The present invention relates to a spray-drying granulation apparatus which accommodates both a spray dryer and a granulator in one chamber.
As the spraying nozzle of spray-drying granulation apparatus, there have conventionally been used a centrifugal pressure nozzle 3 as shown in FIG. 8 which has, at the back of the orifice, a section for generating a strong whirl flow of feed solution (this section is called core, whirl chamber or the like) to provide fine droplets and give a hollow cone-shaped spray pattern for good contact with hot air. Such a centrifugal pressure nozzle is used in, for example, a spray-drying granulation apparatus as shown in FIG. 6 and a spray-drying granulation apparatus as shown in FIG. 7 U.S. Pat. No. 5,044,093) in which a nozzle for binder spraying is provided in the fluidized granulation section in addition to the centrifugal pressure nozzle provided in the spray-drying section. However, there has been used no spraying nozzle capable of giving a narrow spraying angle and droplets having appropriately distributed diameters.
Generally in spray dryers, it is necessary to stabilize the temperature inside the spray-drying chamber, at the start of the operation, in order to prevent the product from scorching caused by excessive heating and assure thermal protection for the subsequent steps. For this purpose, water is sprayed using the same centrifugal pressure nozzle.
In conducting the water spraying, the amount of the water sprayed must correspond to the water content in feed solution. Since the water content in feed solution is generally 30-80% by weight, the amount of water sprayed is also 30-80% by weight based on the amount of feed solution to be sprayed. Consequently, the spraying pressure is reduced to about 10-70% because of the characteristics possessed by the centrifugal pressure nozzle although the pressure varies depending upon the viscosity of feed solution, etc. As a result, the water droplets formed are large making the drying difficult, and they stick to the inner wall of drying chamber in a liquid state and wet the wall. When the feed solution is sprayed successively, the dried powder particles stick thereonto and are solidifed.
Hence, means as shown in FIG. 10 and FIG. 11 have been employed conventionally.
FIG. 10 is a spray-drying granulation apparatus using a plurality of centrifugal pressure nozzles. In this apparatus, a plurality of centrifugal pressure nozzles 25 are provided at the top of the spray-drying chamber and, when water spraying is conducted, water is sprayed with only part of the nozzles 25 to avoid lower-pressure spraying.
FIG. 11 is a spray-drying granulation apparatus using a nozzle 26 for water spraying, independently from a nozzle 27 for feed solution spraying.
In the conventional centrifugal pressure nozzle 3 mentioned above, the spraying angle .alpha. is generally large and 40.degree.-90.degree., as shown in FIG. 8 and FIG. 9. When spraying is made from the centrifugal pressure nozzle 3, the inside 12 of the resulting hollow cone comes to have a negative pressure and the air 17 flows from the outside 16 of the hollow cone to the inside 12. This air movement allows the fine droplets 24 to be concentrated at the inner surface of the cone with the coarse droplets 23 remaining at the outer surface of the cone. The fine droplets 24 are dried easily. Meanwhile, the coarse droplets 23 are hard to dry, remain at the outer surface of the spray pattern, reach the lower portion 18 of the spray-drying section 1 with a large amount of water contained therein, and stick onto and accumulate at the lower portion 18, as shown in FIG. 9. As a result, the yield of product is low and, when the product is a substance of low heat resistance, such as food, medicine, organic compound or the like, the product gives rise to thermal deterioration, resulting in reduced quality. When excessive drying is made in order to avoid the above sticking and accumulation, the water content in each droplet is too low and granulation is not promoted.
In the conventional pressure nozzle having no section for generating a strong whirl flow, at the back of the orifice, the spraying angle can be made small. However, the droplets are too coarse and reach the fluidized granulation section without being substantially dried; as a result, no powder is produced and no fluidized layer is formed.
FIG. 6 is a schematic view of a known spray-drying granulation apparatus. When there is used a centrifugal pressure nozzle 3 as the spraying nozzle of the apparatus, granulation is not promoted unless each of the powder particles formed by spray-drying contain an appropriate amount (about 3-15% by weight) of water. When each powder particle is allowed to contain an appropriate amount of water, the coarse droplets formed by spraying are not sufficiently dried and stick onto the inner wall of the apparatus chamber because the spraying angle of nozzle is large.
Sticking can be avoided by making large the height of the straight cylindrical portion 14 of the apparatus chamber. However, it makes small the granulation rate in the fluidized granulation layer and consequently there is required a large fluidized layer. In the case of, in particular, substances (e.g. amino acid-containing substance) whose granulation is possible in a narrow water content range and which tend to stick outside said water content range, a high granulation rate is employed while avoiding sticking; hence, the selection of operating conditions is difficult.
FIG. 7 is a schematic view of other known spray-drying granulation apparatus. In this apparatus, the height of the straight cylindrical portion 14 is made large in order to solve the above-mentioned sticking problem; as a result, the powder particles have a low water content and the granulation rate is low. Hence, a two-fluid nozzle for binder spraying is provided for improved granulation. This has solved the sticking problem, but has made the apparatus larger and requires a larger space for apparatus installation. Further, use of a two-fluid pressure nozzle for binder spraying requires a large amount of compressed air. Furthermore, the apparatus cost is higher.
Further in the apparatus shown in FIG. 10, the distance between nozzles must be large and part of the nozzles uses no water. Consequently, the droplets do not spread uniformly, giving rise to humidity variation between droplets. Further, the nozzle(s) using no water incurs (incur) clogging.
Further in the apparatus shown in FIG. 11, the nozzle for feed solution spraying is neither cooled nor washed with water and incurs clogging with the feed solution.